The most serious reason is legal. If you formed an LLC or a corporation, that structure exists to separate your personal assets from business liabilities. Courts can set that protection aside when the separation is not real, and the most common evidence they look for is commingled funds. Paying a business expense from a personal account, or a personal expense from the business account, is exactly what that argument is built on. You can hold a perfectly valid LLC on paper and still lose the protection it was formed to provide, and you will find out at the worst possible moment.
The practical reasons compound from there. Tax preparation becomes reconstruction rather than reporting when every transaction has to be sorted by memory at the end of the year, and deductions you cannot substantiate are deductions you effectively did not take. Accounting software connects to one clean account and categorizes automatically instead of pulling in your groceries. Lenders and processors want to see business banking history, and a business with none looks newer than it is. Selling the business later, or bringing in a partner, requires financials that a mixed account cannot produce.
What you actually need is straightforward and the requirements are consistent across banks. Your EIN from the IRS, which is free and takes minutes online. Your formation documents if you registered an entity, or your fictitious firm name filing if you are operating as a sole proprietor under a business name. Government identification. In Nevada, your state business license. Then open a checking account, and separately, a savings account you use only to hold the portion of income set aside for taxes, because tax money in your operating account will get spent.
Two rules make the separation hold. Every dollar the business earns goes into the business account first, without exception. And when you pay yourself, you transfer money from the business account to your personal account as a documented owner draw rather than paying personal bills directly from the business. If you make a mistake, and most people do at some point, record it immediately as an owner draw or a capital contribution rather than leaving it unexplained. A clean record with a few labeled corrections is defensible. A mixed account that nobody can untangle is not.